Chapter 3

Anatomic variants are variations in normal anatomy that can be misinterpreted as pathological conditions. Many anatomic variants are remnant structures from embryological development and fetal circulation, particularly in the atria. Anatomic variants are seen in multiple image planes and persist despite changes in transducer frequency, gain, compression, and depth. Ultrasound artifacts are errors in imaging most commonly due to a violation of the assumptions that are inherent in any ultrasound system. All imaging systems assume that sound travels in a straight line, travels directly back from a reflector, and travels at exactly 1540 m/s through soft tissue. Additionally, it is assumed that the ultrasound beam is very thin, reflections are entirely from structures within the main axis of the beam, and the intensity of reflections is related only to the tissue characteristics of the reflector.1 Artifacts cross known anatomic planes and boundaries and typically disappear with alternate imaging planes and when remedial actions are taken. It is vital to be familiar with the common anatomic variations and ultrasound imaging artifacts to ensure accurate echocardiographic interpretation and to avoid unnecessary interventions.2

The atria and the sinus venosus evolve in the 4th week of embryonic development. Initially, the sinus venosus receives venous blood from left and right sinus horns (Figure 3–1A and B). In time, the veins to the left sinus horn are obliterated and the remnants become the coronary sinus. The right sinus horn, on the other hand, enlarges and forms the smooth-walled part of the right atrium (RA), known as the sinus venarum. As the RA expands, the sinus venarum displaces the trabeculated tissue of the primitive RA into the periphery and into the right atrial appendage (which may have prominent pectinate muscles). Right and left venous valves mark the junction of the original sinus venarum and the primitive RA. The left venous valve disappears as it fuses with the developing atrial septum. The right venous valve of the right sinus venosus horn develops inferiorly into (1) the valve of the inferior vena cava (IVC) or the eustachian valve, which directs fetal blood flow from the IVC across the foramen ovale, and (2) the valve to the coronary sinus or the thebesian valve (Figure 3–2). Superiorly, the convergence of the smooth sinus venarum and the trabeculated RA is the crista terminalis. Concurrently, the atrial septum forms with migration of the septum primum to obliterate the ostium primum, followed by the migration of the septum secundum to cover the ostium secundum. This migration leads to the characteristic thin-walled appearance of the foramen ovale, with incomplete septation leading to the possibility of a patent foramen ovale.3

###### Figure 3-1.

A: Illustration of the right and left sinus venosus horns as they join the developing heart.3 B: The remnant of ...

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